Since liquid crystal displays are lighter and more compact display devices than CRTs, they find extensive application in computers, electronic calculators, clocks, and watches. The principle of operation of liquid crystal displays depends on a change in an optical property, such as interference, scattering, diffraction, optical rotation, or absorption, of a liquid crystal material. This change is caused by a variation in orientation of the liquid crystal molecules or a phase transition in response to application of an external field such as electric field or heat.
Generally, a liquid crystal display comprises a pair of substrates with a given spacing which is maintained at 1 to tens of micrometers. A liquid crystal material is held between these substrates, thus forming a liquid crystal panel. At least one of the substrates has transparency. Electrodes are formed on both or one of the substrates. Using these electrodes, an electric field is applied to the liquid crystal material to control the orientation of the liquid crystal molecules in each different pixel within the substrate plane, thus controlling the amount of light transmitted through the liquid crystal panel. In this way, a desired image is displayed. The liquid crystal display is constructed differently according to which is the above-described optical properties is utilized, i.e., depending on the mode of operation. For example, polarizing plates are mounted on the Outside of the liquid crystal panel.
To date, twisted-nematic (TN) liquid crystal displays and supertwisted-nematic (STN) liquid crystal displays enjoy wide acceptance. These kinds of liquid crystal displays make use of optical properties of liquid crystal materials such as optical rotation and interference of birefringent light. Both kinds require polarizing plates.
Where an image is displayed by the above-described liquid crystal display, numerous pixels must be controlled at the same time. For this purpose, various methods have been proposed. Among them, active matrix driving is used widely because it is a method capable of displaying image with high information content and high image quality. In particular, nonlinear active devices such as diodes or transistors are arranged at pixels. The individual pixels are electrically isolated from each other. Interference with unwanted signals is prevented. Thus, high image quality is accomplished. In this method, each pixel can be regarded as a capacitor to which an electrical switch is connected. Accordingly, the switches are turned on and off according to the need. As a result, electric charge can be made to go into and out of the pixels. If the switches are turned off, electric charge can be retained in the pixels and so the pixels can retain memory.